a http-equiv="Content-Type" content="text/html; charset=iso-8859-1">Prevailing Winds

Flight Environment



Since the setting is fixed to the earthby gravity and rotates through the earth, there would be no circulation if some force did notupset the atmosphere"s equilibrium. The heater of the earth"s surface ar by the sunlight isthe force responsible for developing the circulation the does exist.

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Because the the curvature of the earth,the most straight rays that the sun strike the earth in the vicinity the the equator resultingin the best concentration the heat, the largest feasible amount the radiation, and themaximum heater of the setting in this area that the earth. In ~ the very same time, thesun"s beam strike the earth at the poles at a an extremely oblique angle, leading to a muchlower concentration of heat and much much less radiation so the there is, in fact, an extremely littleheating that the atmosphere over the poles and also consequently very cold temperatures.

Cold air, being more dense, sink andhot air, being less dense, rises. Consequently, the rising warmth air at the equatorbecomes even less thick as the rises and also its press decreases. An area of lowpressure, therefore, exists over the equator.


Warm air rises till it will acertain elevation at which it starts to spill over into surrounding areas. In ~ thepoles, the cold dense air sinks. Wait from the upper levels that the setting flowsin on top of it enhancing the weight and also creating an area that high pressure at the poles.

The air the rises in ~ the equator walk notflow straight to the poles. As result of the rotation that the earth, over there is a build up that airat about 30° north latitude. (The exact same phenomenon wake up in the southerly Hemisphere). Some of the waiting sinks, bring about a belt the high-pressure at this latitude.


The sinking air reaches the surfaceand flows north and also south. The air that flows southern completes one cabinet of theearth"s circulation pattern. The air that flows phibìc becomes component of another cellof circulation in between 30° and 60° phibìc latitude. At the same time, the sinkingair at the phibìc pole operation south and collides through the air moving north indigenous the 30°high pressure area. The colliding wait is compelled upward and an area of short pressureis developed near 60° north. The third cell circulation sample is developed betweenthe north pole and 60° north.

Because the the rotation that the earthand the coriolis force, air is deflected come the appropriate in the northern Hemisphere. Asa result, the movement of wait in the polar cell circulation produces the polar easterlies. In the circulation cell the exists between 60° and 30° north, the activity ofair produce the prevailing westerlies. In the tropic circulation cell, thenortheast trade winds are produced. These are the so-called permanent wind systemsof the each.

Since the earth rotates, the axis is tilted, and also there is an ext land massive in the north hemisphere 보다 in the southerly hemisphere, the actual an international pattern is much much more complicated. Instead of one huge circulation in between the poles and also the equator, there space three circulations...Hadley cell
- short latitude air activity toward the equator that v heating, rises vertically, with poleward movement in the upper atmosphere. This develops a convection cell the dominates tropical and also sub-tropical climates.Ferrel cell - A mid-latitude median atmospheric circulation cell for ubraintv-jp.com called by Ferrel in the 19th century. In this cell the air flows poleward and eastward close to the surface and equatorward and westward at higher levels.Polar cell - air rises, diverges, and travels towards the poles. Once over the poles, the air sinks, creating the polar highs. In ~ the surface air diverges external from the polar highs. Surface ar winds in the polar cell are easterly (polar easterlies).


There room two main pressures which influence the movement of waiting in the upper levels. The pressure gradient causes the waiting to move horizontally, forcing the air straight from a region of high push to a an ar of short pressure. The Coriolis force, however, deflects the direction the the flow of the waiting (to the appropriate in the north Hemisphere) and causes the waiting to flow parallel come the isobars.

Winds in the upper levels will blow clockwise roughly areas the high pressure and also counterclockwise about areas of low pressure.

The rate of the wind is determined by the pressure gradient. The winds room strongest in areas where the isobars room close together.


Surface friction plays critical role in the speed and direction of surface winds. As a an outcome of the slowing under of the air together it moves end the ground, wind speed are less than would be meant from the press gradient on the ubraintv-jp.com map and the direction is changed so the the wind blows across the isobars right into a facility of short pressure and out that a center of high pressure.


The result of friction generally does no extend an ext than a couple of thousand feet right into the air. In ~ 3000 feet over the ground, the wind blows parallel come the isobars through a rate proportional to the press gradient.

Even allowing for the effects of surface ar friction, the winds, locally, perform not always show the speed and direction that would be expected from the isobars on the surface ubraintv-jp.com map. This variations are usually as result of geographical features such as hills, hills and huge bodies the water. Other than in mountainous regions, the effect of terrain attributes that cause local sports in wind extends usually no greater than about 2000 feet over the ground.


Land and also sea breezes are resulted in by the differences in temperature over land and also water. The sea breeze occurs throughout the day once the floor area heats an ext rapidly than the water surface. This outcomes in the press over the soil being lower than that over the water. The push gradient is often strong enough because that a wind to blow from the water to the land.


The floor breeze blows at night as soon as the land i do not care cooler. Climate the wind blows towards the warm, low-pressure area end the water.

Land and sea breezes are really local and influence only a narrow area along the coast.


Hills and valleys significantly distort the airflow connected with the prevailing press system and also the pressure gradient. Strong up and also down drafts and also eddies develop as the air operation up over hills and down right into valleys. Wind direction transforms as the waiting flows around hills. Occasionally lines of hills and mountain varieties will act as a barrier, holding earlier the wind and also deflecting the so that it operation parallel come the range. If over there is a happen in the mountain range, the wind will rush v this pass as with a tunnel with significant speed. The airflow deserve to be expected to stay turbulent and also erratic for some distance as it flows out of the hilly area and also into the flatter countryside.


Daytime heating and also nighttime cooling the the hilly slopes result in day to night variations in the airflow. At night, the political parties of the hills cool by radiation. The wait in contact with them becomes cooler and therefore denser and also it blows under the slope right into the valley. This is a katabatic wind (sometimes additionally called a mountain breeze). If the slopes are covered through ice and snow, the katabatic wind will certainly blow, not just at night, but also during the day, transferring the cold dense air into the warmer valleys. The slopes the hills not covered by snow will be warmed throughout the day. The waiting in call with them becomes warmer and also less dense and, therefore, operation up the slope. This is one anabatic wind (or valley breeze).

In like the mountain areas, local distortion that the airflow is even more severe. Rocky surfaces, high ridges, sheer cliffs, steep valleys, all integrate to develop unpredictable circulation patterns and also turbulence.


THE mountain WAVE

Air flowing across a mountain range usually rises fairly smoothly up the slope of the range, but, as soon as over the top, it pours under the various other side with considerable force, bouncing up and also down, developing eddies and also turbulence and additionally creating an effective vertical waves the may expand for great distances downwind of the mountain range. This phenomenon is recognized as a mountain wave. Keep in mind the up and down drafts and the rotating eddies created downstream.

If the waiting mass has a high moisture content, clouds of really distinctive appearance will certainly develop.

Cap Cloud. Orographic lift causes a cloud to type along the peak of the ridge. The wind tote this cloud down along the leeward slope wherein it dissipates through adiabatic heating. The base of this cloud lies near or listed below the peaks of the ridge; the top might reach a couple of thousand feet over the peaks.

Lenticular (Lens Shaped) Clouds form in the wave crests aloft and also lie in bands that may expand to well over 40,000 feet.

Rotor Clouds form in the rojo eddies downstream. They resemble a long line the stratocumulus clouds, the bases of i beg your pardon lie below the mountain peaks and the tops of which might reach come a significant height over the peaks. Periodically these clouds develop into thunderstorms.


The clouds, being really distinctive, deserve to be seen from a an excellent distance and provide a visible warning that the mountain wave condition. Unfortunately, sometimes they are embedded in other cloud systems and also are surprise from sight. Occasionally the air mass is very dry and the clouds perform not develop.

The severity of the mountain wave and also the elevation to which the disturbance that the wait is influenced is dependence on the stamin of the wind, its angle to the selection and the security or instability of the air. The many severe hill wave problems are developed in solid airflows that space blowing at ideal angles come the variety and in stable air. A jet currently blowing virtually perpendicular come the mountain variety increases the severity of the wave condition.

The hill wave phenomenon is not restricted only to high hill ranges, such as the Rockies, but is also present come a lesser level in smaller hill systems and even in currently of small hills.

Mountain waves current problems to pilots for number of reasons:

Vertical Currents. Downdrafts of 2000 feet every minute are common and also downdrafts as an excellent as 5000 feet every minute have been reported. They happen along the bottom slope and are many severe in ~ a height equal to that of the summit. One airplane, caught in a downdraft, could be required to the ground.

Turbulence is usually exceptionally severe in the air layer between the ground and the tops of the rotor clouds.

Wind Shear. The wind speed varies dramatically in between the crests and also troughs of the waves. That is usually many severe in the tide nearest the hill range.

Altimeter Error. The rise in wind speed outcomes in one accompanying diminish in pressure, which in turn affects the accuracy of the press altimeter.

Icing. The freezing level varies significantly from crest to trough. Serious icing deserve to occur because of the large supercooled droplets sustained in the solid vertical currents.

When flying over a hill ridge where wave problems exist: (1) stop ragged and irregular shame clouds—the rarely often, rarely shape shows turbulence. (2) strategy the hill at a 45-degree angle. That you must suddenly decide to turn back, a fast turn have the right to be made away from the high ground. (3) protect against flying in cloud on the mountain crest (cap cloud) because of strong downdrafts and also turbulence. (4) permit sufficient height to clean the highest ridges through altitude to preventive to protect against the downdrafts and eddies top top the downwind slopes. (5) always remember that your altimeter can read over 3000 ft. In error on the high side in mountain wave conditions.


A gust is a rapid and irregular fluctuation of differing intensity in the upward and downward activity of air currents. It may be connected with a rapid adjust in wind direction. Gusts are resulted in by mechanical disturbance that results from friction in between the air and the ground and also by the unequal heater of the earth"s surface, specifically on hot summer afternoons.


A squall is a sudden rise in the stamin of the wind of much longer duration 보다 a gust and also may be brought about by the passage of a fast moving cold prior or thunderstorm. Like a gust, it might be add by a rapid readjust of wind direction.


Diurnal (daily) variation of wind is brought about by solid surface heating during the day, which causes turbulence in the lower levels. The an outcome of this turbulence is that the direction and also speed of the wind in ~ the greater levels (e.g., 3000 feet) has tendency to be transferred to the surface. Because the wind direction at the higher level is parallel come the isobars and also its speed is higher than the surface wind, this transfer causes the surface ar wind come veer and also increase in speed.

At night, there is no surface ar heating and therefore much less turbulence and also the surface ar wind tends to resume its normal direction and speed. The backs and decreases. Check out VEERING and also BACKING section below for an ext info.


Friction between the relocating air mass and also surface attributes of the earth (hills, mountains, valleys, trees, buildings, etc.) is responsible for the swirling vortices the air commonly called eddies. Lock vary significantly in size and intensity relying on the size and roughness of the surface ar obstruction, the speed of the wind and also the level of security of the air. They can spin in one of two people a horizontal or vertical plane. Turbulent air and strong winds produce an ext vigorous eddies. In stable air, eddies often tend to quickly dissipate. Eddies created in mountainous areas are particularly powerful.


The bumpy or choppy up and also down movement that signifies the visibility of eddies makes it challenging to store an aircraft in level flight.


Dust devils space phenomena that occur quite frequently on the hot dry plains of mid-western north America. They deserve to be of adequate force to current a risk to pilots of light airplanes flying at short speeds.

They are tiny heat short that kind on clear warm days. Offered a steep slide out rate led to by cool waiting aloft end a hot surface, tiny horizontal waiting movement, couple of or no clouds, and also the noonday sun heating flat arid floor surfaces come high temperatures, the wait in contact with the ground i do not care super-heated and highly unstable. This surface ar layer of air builds till something triggers an increase movement. As soon as started, the warm air rises in a column and also draws more hot air into the basic of the column. Circulation begins around this heat low and also increases in velocity till a tiny vigorous whirlwind is created. Dust devils are usually of brief duration and also are therefore named because they space made clearly shows by the dust, sand and debris that they choose up indigenous the ground.

Dust devils posture the biggest hazard close to the ground wherein they are many violent. Pilots proposing to land ~ above superheated runways in locations of the mid-west wherein this phenomenon is common should scan the airport for dust swirls or grass spirals that would suggest the existence of this hazard.


Tornadoes space violent, one whirlpools the air linked with significant thunderstorms and are, in fact, an extremely deep, focused low-pressure areas. They are shaped like a tunnel hanging out of the cumulonimbus cloud and are dark in appearance due to the dust and also debris sucked into their whirlpools. They range in diameter from around 100 feet come one half mile and move end the ground at speed of 25 come 50 knots. Their path over the floor is usually only a couple of miles long although tornadoes have been reported to cut destructive swaths as long as 100 miles. The an excellent destructiveness of tornadoes is brought about by the very low pressure in your centers and the high wind speeds, i beg your pardon are deemed to it is in as an excellent as 300 knots.


Wind speeds because that aviation purposes are express in knots (nautical miles every hour). In the ubraintv-jp.com reports on us public radio and television, however, wind speed are provided in miles every hour when in Canada speeds are provided in kilometers every hour.

In a discussion of wind direction, the compass allude from i m sorry the wind is blowing is thought about to it is in its direction. Therefore, a north wind is one that is blowing from the north in the direction of the south. In aviation ubraintv-jp.com reports, area and also aerodrome forecasts, the wind is constantly reported in levels true. In ATIS broadcasts and in the information given by the tower because that landing and also take-off, the wind is report in degrees magnetic.


The wind veers once it alters direction clockwise. Example: The surface ar wind is blowing native 270°. At 2000 feet it is blowing native 280°. The has adjusted in a right-hand, or clockwise, direction.

The wind backs when it changes direction anti-clockwise. Example: The wind direction in ~ 2000 feet is 090° and also at 3000 feet is 085°. That is an altering in a left-hand, or anti-clockwise, direction.

In a lower from numerous thousand feet above the ground come ground level, the wind will typically be uncovered to earlier and likewise decrease in velocity, as the impact of surface ar friction i do not care apparent. In a rise from the surface to several thousand feet AGL, the wind will certainly veer and also increase.

At night, surface cooling reduce the eddy activity of the air. Surface winds will ago and decrease. Conversely, during the day, surface ar heating rises the eddy motion of the air. Surface ar winds will veer and increase as more powerful winds aloft mix come the surface. Watch DIURNAL VARIATIONS section over for more info.


Wind shear is the suddenly tearing or shearing effect encountered follow me the leaf of a region in which there is a violent adjust in wind rate or direction. It can exist in a horizontal or vertical direction and also produces churning motions and also consequently turbulence. Under some conditions, wind direction changes of as lot as 180 degrees and also speed alters of as much as 80 knots have been measured.


The impact on airplane performance the encountering wind shear derives native the reality that the wind can adjust much faster than the plane mass have the right to be increased or decelerated. Major wind shears have the right to impose penalties on one airplane"s performance the are beyond its capabilities to compensate, especially throughout the crucial landing and also take-off step of flight.

In cruising Flight

In cruising flight, wind shear will most likely be encountered in the change zone in between the push gradient wind and the distorted neighborhood winds in ~ the reduced levels. It will additionally be encountered once climbing or descending with a temperature inversion and also when passing v a frontal surface. Wind shear is also associated through the jet stream. Airplanes encountering wind shear may experience a succession of updrafts and downdrafts, reductions or gains in headwind, or windshifts the disrupt the developed flight path. The is not commonly a major problem because altitude and also airspeed margins will be sufficient to counteract the shear"s adverse effects. On occasion, however, the wind shear might be severe sufficient to cause an abrupt increase in load factor, which might stall the airplane or inflict structural damage.


Near the Ground

Wind shear, encountered near the ground, is more serious and potentially an extremely dangerous. There space four typical sources of short level wind shear: thunderstorms, frontal activity, temperature inversions and strong surface winds passing about natural or manmade obstacles.

Frontal Wind Shear. Wind shear is normally a difficulty only in fronts with steep wind gradients. If the temperature difference across the front in ~ the surface ar is 5°C or an ext and if the former is moving at a speed of about 30 knots or more, wind shear is likely to it is in present. Frontal wind shear is a phenomenon associated with rapid moving cold fronts but can be present in heat fronts together well.


Thunderstorms. Wind shear, connected with thunderstorms, occurs together the result of 2 phenomena, the gust front and also downbursts. Together the thunderstorm matures, strong downdrafts develop, strike the ground and spread out horizontally follow me the surface well in development of the thunderstorm itself. This is the gust front. Winds can readjust direction by as lot as 180° and also reach speed as an excellent as 100 knots as much as 10 mile ahead the the storm. The downburst is really intense localized downdraft flowing out of a thunderstorm. The power of the downburst can exceed plane climb capabilities. The downburst (there space two types of downbursts: macrobursts and microbursts) normally is much closer come the thunderstorm 보다 the gust front. Dust clouds, roll clouds, extreme rainfall or virga (rain that evaporates before it reaches the ground) are as result of the opportunity of downburst activity but there is no way to accurately predict its occurrence.


Temperature Inversions. Overnight cooling creates a temperature turning back a couple of hundred feet above the soil that deserve to produce far-ranging wind shear, especially if the inversion is coupled through the low-level jet stream.

As a nocturnal inversion develops, the wind shear close to the peak of the inversion increases. It normally reaches its maximum speed quickly after midnight and also decreases in the morning as daytime heating dissipates the inversion. This phenomenon is well-known as the low-level nocturnal jet stream. The low level jet stream is a sheet of strong winds, hundreds of miles long, thousands of miles large and hundreds of feet special that creates over level terrain such together the prairies. Wind speeds of 40 knots space common, however greater speeds have been measured. Short level jet streams room responsible because that hazardous short level shear.


As the reverse dissipates in the morning, the shear aircraft and gusty winds move closer come the ground, causing windshifts and also increases in wind speed close to the surface.

Surface Obstructions. The irregular and turbulent flow of air roughly mountains and also hills and also through hill passes causes serious wind shear problems for plane approaching come land at airports near hill ridges. Wind shear is a phenomenon connected with the mountain wave. Together shear is practically totally unpredictable but should be meant whenever surface ar winds room strong.

Wind shear is additionally associated through hangars and large buildings in ~ airports. Together the wait flows about such huge structures, wind direction changes and wind speed increases causing shear.

Wind shear occurs both horizontally and also vertically. Vertical shear is most common near the ground and also can pose a serious danger to airplanes during take-off and landing. The airplane is flying at lower speeds and in a relatively high traction configuration. Over there is little altitude available for recovering and also stall and maneuver margins are at their lowest. An airplane encountering the wind shear phenomenon may experience a large loss of airspeed due to the fact that of the sudden adjust in the relative airflow together the plane flies right into a new, moving air mass. The abrupt autumn in airspeed may result in a stall, developing a dangerous situation when the aircraft is only a couple of hundred feet off the floor and an extremely vulnerable.


Narrow bands that exceedingly high speed winds are recognized to exist in the greater levels that the atmosphere at altitudes ranging from 20,000 come 40,000 feet or more. They are well-known as jet streams. As plenty of as three major jet streams might traverse the phibìc American continent at any kind of given time. One lies across Northern Canada and one across the U.S. A 3rd jet stream may be as much south as the north tropics yet it is rather rare. A jet present in the mid latitudes is normally the strongest.


The jet stream appears to be closely connected with the tropopause and also with the polar front. It commonly forms in the break between the polar and the tropic tropopause where the temperature gradients space intensified. The median position that the jet stream shears south in winter and also north in summer v the seasonal hike of the polar front. Since the troposphere is depth in summer than in winter, the tropopause and the jets will nominally it is in at higher altitudes in the summer.

Long, strong jet streams are usually also associated with well-developed surface lows in ~ deep upper troughs and also lows. A low arising in the wave along the frontal surface ar lies southern of the jet. Together it deepens, the low moves close to the jet. As it occludes, the low moves phibìc of the jet, which crosses the frontal system, close to the suggest of occlusion. The jet flows around parallel come the front. The subtropical jet currently is not linked with fronts however forms because of strong solar heating in the equatorial regions. The ascending air transforms poleward at an extremely high levels however is deflected through the Coriolis pressure into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams circulation from west to east and may encircle the entire hemisphere. More often, because they space stronger in some locations than in others, they break up right into segments some 1000 come 3000 nautical miles long. They room usually about 300 nautical miles broad and might be 3000 come 7000 feet thick. This jet present segments move in an easterly direction complying with the activity of press ridges and also troughs in the upper atmosphere.

Winds in the central core that the jet stream space the strongest and may reach speeds as great as 250 knots, return they room generally between 100 and also 150 knots. Wind speeds decrease toward the outer edges the the jet stream and also may be blowing at just 25 knots there. The rate of to decrease of wind speed is significantly greater ~ above the north edge than on the southerly edge. Wind speed in the jet stream are, on average, considerably stronger in winter than in summer.

Clear wait Turbulence. The most probable place to suppose Clear Air disturbance (CAT) is just above the central core that the jet stream near the polar tropopause and just below the core. Clean air turbulence does not occur in the core. CAT is encountered more frequently in winter once the jet stream winds room strongest. Nevertheless, CAT is not constantly present in the jet stream and, due to the fact that it is random and also transient in nature, that is practically impossible come forecast.

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Clear air turbulence might be connected with other ubraintv-jp.com patterns, especially in wind shear associated with the sharply bent contours of strong lows, troughs and also ridges aloft, at or listed below the tropopause, and in locations of solid cold or heat air advection. Mountain waves produce severe CAT that may expand from the mountain crests to as high together 5000 feet over the tropopause. Because severe CAT does posture a risk to airplanes, pilots should try to stop or minimize encounters v it. This rules of thumb may assist avoid jet streams with strong winds (150 knots) at the core. Strong wind shears room likely above and below the core. CAT in ~ the jet present is more intense over and come the lee of mountain ranges. If the 20-knot isotachs (lines joining locations of same wind speeds) room closer than 60 nautical miles on the charts showing the places of the jet stream, wind shear and also CAT room possible.

Curving jet streams are likely to have turbulent edges, particularly those that curve about a deep pressure trough. Once moderate or significant CAT has actually been report or is forecast, readjust speed to turbulent air speed immediately on encountering the very first bumpiness or even before encountering the to protect against structural damage to the airplane.

The locations of CAT are usually shallow and narrow and also elongated through the wind. If jet stream turbulence is encountered with a tail wind or head wind, a rotate to the best will uncover smoother air and much more favorable winds. If the CAT is encountered in a crosswind, that is not so crucial to adjust course together the turbulent area will certainly be narrow.